The need to present an undistorted image to the viewer of a cathode ray tube (CRT) presents problems to the designer of the control apparatus. The image is formed by scanning an electron beam over the tube face, a face which may be curved, but does not have the same radius as the apparent radius of the electron beam. In a single beam tube, this results in what is known as pin-cushion distortion, a pinching inwards of the scan at the center of the edges of the image, and an elongation at the corners. Because the gun in a single beam tube is aligned along a central axis of the tube pin-cushion distortion is symetrical about a center point of te screen.
Distortion of the image is aggravated in a three beam color tube, as of necessity the three guns are displaced from each other and each color produces its own displaced pin-cushion raster on the screen. The three beams may, without correction, converge on one spot in the center of the screen but equal horizontal and vertical deflection of the three beams results in a divergence of the beams as well as a displaced pin-cushion effect.
Consequently, two distortions have to be corrected before an image is acceptable to a viewer; first the pin-cushion effect and secondly the convergence of the three beams so that they present an aligned image.
Both pin-cushion and convergence distortion are well known in the art and there have been many solutions proposed and used for the correction of distortions in both monochrome and color CRTs. In "Colour Television Theory" by G. H. Hutson published by McGraw-Hill, London, chapters 6 and 7 deal in some detail with convergence and raster shape correction circuitry.
Many of the solutions used to correct distortion derive a correction waveform from either the field deflection current or the line deflection current which is adjusted by a series of potentiometers to achieve the best correction. The correction waveform is then applied to a set of correction or convergence coils usually placed on the gun side of the main deflection coils. While analog correction techniques can give a satisfactory picture in the center and a large area of the screen, there tends to be a portion of the screen around the edges in which the images in a three gun color tube diverge. This divergence in a CRT used for domestic viewing may be acceptable and not easily noticeable when the image is viewed from a distance of 2 meters or more, but when a CRT is used for displaying alphanumeric data, and where the viewer is the operator of a display terminal viewing from a distance of under 1 meter, divergence at the edges becomes a significant factor.
The use of digital techniques to correct for distortion is known as is shown in UK Patent Serial No. 1,371,045 in which pin-cushion or trapezium distortion is corrected by using a correction waveform derived through a decoder from a stepping register. The digital system described in UK Patent No. 1,371,045 is applied only to a monochrome CRT and is not susceptible to adjustment when the CRT is in use.